Substrate support for chucking of mask for deposition processes

ABSTRACT

Embodiments of the disclosure include methods and apparatus for electrostatically coupling a mask to a substrate support in a deposition chamber. In one embodiment, a substrate support is disclosed that includes a substrate receiving surface, a recessed portion disposed about a periphery of the substrate receiving surface, an electrostatic chuck disposed below the substrate receiving surface, and a plurality of compressible buttons disposed within a respective opening formed in the recessed portion that form an electrical circuit with the electrostatic chuck.

BACKGROUND Field

Embodiments of the disclosure relate to methods and apparatus forsecuring a mask utilized in a deposition process, such as a plasmaenhanced chemical vapor deposition (PECVD) process or an atomic layerdeposition (ALD) process used in the manufacture of electronic devices.In particular, embodiments of the disclosure relate to securing of ametallic shadow mask utilized in an encapsulation process in themanufacture of organic light emitting diode (OLED) display devices withPECVD and/or ALD processes.

Description of the Related Art

Organic light emitting diodes (OLEDs) are used in the manufacture oftelevision screens, computer monitors, mobile phones, other hand-helddevices, etc. for displaying information. A typical OLED may includelayers of organic material situated between two electrodes that are alldeposited on a substrate in a manner to form a matrix display panelhaving individually energizable pixels. The OLED is generally placedbetween two glass panels, and the edges of the glass panels are sealedto encapsulate the OLED therein.

There are many challenges encountered in the manufacture of such displaydevices. In some fabrication steps, the OLED material is encapsulated inone or more layers to prevent moisture from damaging the OLED material.During these processes, one or more masks are utilized to shieldportions of the substrate that do not include the OLED material. Themasks are carefully positioned relative to the substrate in order tocontrol deposition. The masks utilized in these processes are typicallymetals or metal alloys having a relatively low coefficient of thermalexpansion. However, during processing, the mask is typically misalignedand/or mispositioned relative to the substrate. The misalignment ormispositioning generally means the mask is not in proximity to thesubstrate. When the mask is misaligned and/or mispositioned as such, ashadowing effect occurs on the substrate and/or an errant coating isformed under the mask. One or both of the aforementioned issues createproblems, one of them being a “mura effect” or “clouding” of portions ofthe final display product.

Therefore, there is a need for methods and apparatus for improvedpositioning of masks during the formation of OLED display devices.

SUMMARY

Embodiments of the disclosure include methods and apparatus forelectrostatically coupling a mask to a substrate support in a depositionchamber. In one embodiment, a substrate support is disclosed thatincludes a substrate receiving surface, a recessed portion disposedabout a periphery of the substrate receiving surface, an electrostaticchuck disposed below the substrate receiving surface, and a plurality ofcompressible buttons disposed within a respective opening formed in therecessed portion that form an electrical circuit with the electrostaticchuck.

In another embodiment, a substrate support is disclosed that includes abody comprising a conductive material, a substrate receiving surfacecomprising a dielectric layer adhered to the body, wherein the bodyincludes a recessed portion disposed about a periphery of the substratereceiving surface, an electrostatic chuck disposed below the substratereceiving surface, and a plurality of compressible buttons disposedwithin a respective opening formed in the recessed portion that form anelectrical circuit with the electrostatic chuck.

In another embodiment, a substrate support is disclosed that includes abody comprising a conductive material, a substrate receiving surfacecomprising a dielectric layer adhered to the body, wherein the bodyincludes a recessed portion disposed about a periphery of the substratereceiving surface, and the substrate receiving surface comprises agroove pattern, an electrostatic chuck disposed in the dielectric layerbelow the substrate receiving surface, and a plurality of compressiblebuttons disposed within a respective opening formed in the recessedportion that form an electrical circuit with the electrostatic chuck.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features can be understoodin detail, a more particular description of embodiments of thedisclosure, briefly summarized above, may be had by reference to theembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments and are therefore not to be considered limiting ofits scope, for the disclosure may admit to other equally effectiveembodiments.

FIG. 1 is a schematic cross-sectional view of a plasma enhanced chemicalvapor deposition (PECVD) chamber according to one embodiment.

FIG. 2 is an exploded isometric view of interior chamber components usedin the chamber body of the PECVD chamber of FIG. 1.

FIG. 3 is a sectional side view of one embodiment of a portion of thesubstrate support as disclosed herein.

FIGS. 4A-4C are schematic sectional views of portions of the substratesupport and electrostatic chuck with different electricalconfigurations.

FIG. 5A is a sectional side view of a portion of a substrate supportaccording to another implementation.

FIG. 5B is an enlarged view of the substrate support of FIG. 5A.

FIG. 6 is a sectional view of a portion of the body of the substratesupport and the mask frame showing one embodiment of a compressiblebutton.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Embodiments of the disclosure include methods and apparatus forelectrically grounding a shadow mask for use in a deposition chamber.The mask may be utilized in a plasma-enhanced chemical vapor deposition(PECVD) process chamber that is operable to align the mask with respectto a substrate, position the mask on the substrate, and deposit anencapsulation layer on an OLED material formed on the substrate. Theembodiments described herein may be used with other types of processchambers and are not limited to use with PECVD process chambers. Theembodiments described herein may be used with other types of depositionprocesses and are not limited to use for encapsulating OLED's formed onsubstrates. The embodiments described herein may be used with varioustypes, shapes, and sizes of masks and substrates. Furthermore, asuitable chamber that may benefit from the masks disclosed herein isavailable from AKT America, Inc., Santa Clara, Calif., which is asubsidiary of Applied Materials, Inc., Santa Clara, Calif.

FIG. 1 is a schematic cross-sectional view of a PECVD chamber 100according to one embodiment. The PECVD chamber 100 includes a chamberbody 102 having an opening 104 through one or more walls to permit oneor more substrates 106 and a mask 108 to be inserted therein. Thesubstrate 106, during processing, is disposed on a substrate support 110opposite a diffuser 113. The substrate support 110 includes a substratereceiving surface 112 having a plurality of grooves 115 formed therein.The plurality of grooves 115 are utilized to provide a backside gas tothe substrate 106 positioned thereon. The backside gas from theplurality of grooves 115 enables temperature uniformity of the substrate106 positioned on the substrate support 110. The backside gases areprovided by a gas source 129. The gases are provided to the grooves 115by a conduit disposed in a stem 130 that supports the substrate support110.

The diffuser 113 has one or more openings 114 formed therethrough topermit a processing gas or a processing material to enter a processingspace 116 between the diffuser 113 and the substrate 106. The processingmaterial may be a silicon containing gas, an aluminum containing gas, apolymer material, as well a carrier gases and/or reactive gases thatform an encapsulation layer on OLED devices formed on the substrate 106.

The substrate 106 may be used to form an OLED display where OLED's areformed on the surface of the substrate 106 by sequential depositionprocesses in the PECVD chamber 100. The substrate 106 may be glasssubstrate, a polymer substrate, or other suitable material for formingelectronic devices. The substrate 106 may be rigid or flexible. Thesubstrate 106 may be used to form a single display or multiple displays.Each display includes a plurality of OLEDs coupled to an electricalcontact layer formed about a perimeter of each display. Duringmanufacture, the OLED portion of each display is encapsulated in one ormore layers to protect the OLED's from the environment. The layers maycomprise one or a combination of silicon nitride, aluminum oxide, and/ora polymer material. The encapsulation material may be deposited by aPECVD process in the PECVD chamber 100. The mask 108 is used to shieldthe electrical contact layer of the OLEDs during deposition of theencapsulation material. The mask 108 includes a mask frame 118 and aplurality of open areas or slots 121. Each slot 121 may be sizedaccording the size of the OLED portion of each display. Theencapsulation material is deposited on the OLED portion of each displaythrough the slots 121. Outward of and between each slot 121 is a strip120 that shields the electrical contact layer during the encapsulationprocess.

The substrate support 110 also includes a heater 117 that heats thesubstrate 106 positioned thereon. The substrate support 110 alsoincludes an electrode 119. The electrode 119 is utilized to chuck thesubstrate 106 and/or the mask 108 to the substrate support 110. Theelectrode 119 is configured as an electrostatic chuck that is utilizedto fix the mask 108 relative to the substrate 106. The electrode 119 iscoupled to a power source 127.

One or more compressible couplers 125 may be utilized to align the mask108 relative to the substrate support 110. The mask 108, including themask frame 118 and the strips 120, are made of a conductive material,such as a metallic alloy material. In one embodiment, the mask 108comprises a material having a low coefficient of thermal expansion.Examples of metallic alloys include KOVAR® alloys (Ni—Co) and INVAR®alloys (Ni—Fe). The substrate support 110 may be made of an electricallyconductive material, such as aluminum. In one implementation, thecompressible couplers 125 may be made of a conductive material thatfunctions to electrically couple the mask 108 to the substrate support110. Thus, electrons that accumulate on the substrate 106 and/or themask 108 during PECVD processing may be transferred to ground potentialthrough or on the mask 108, the compressible couplers 125 and thesubstrate support 110.

For processing, the mask 108 is initially inserted into the PECVDchamber 100 through the opening 104 and disposed upon multiple motionalignment elements 122. The substrate support 110 is disposed on thestem 130 that is coupled to an actuator 123. The elevation of thesubstrate support 110 in the PECVD chamber 100 may be controlled by theactuator 123. When the substrate support 110 is lowered to a leveladjacent to the opening 104, the substrate 106 may be inserted thoughthe opening 104 and disposed upon multiple lift pins 124 that extendthrough the substrate support 110. The substrate support 110 then raisesto meet the substrate 106 so that the substrate 106 is supported on thesubstrate support 110. The substrate 106 may be aligned while on thesubstrate support 110.

Once the substrate 106 is aligned on the substrate support 110, one ormore visualization systems 126 determine whether the mask 108 isproperly aligned over the substrate 106. If the mask 108 is not properlyaligned, one or more actuators 128 move one or more motion alignmentelements 122 to adjust the location of the mask 108 relative to thesubstrate support 110. The one or more visualization systems 126 maythen recheck the alignment of the mask 108 to verify alignment.

Once the mask 108 is properly aligned over the substrate 106, the mask108 is lowered onto the substrate 106, and the substrate support 110 israised until a shadow frame 132 contacts the mask 108. The shadow frame132, prior to resting on the mask 108, is disposed in the chamber body102 on a ledge 134 that extends from one or more interior walls of thechamber body 102. The substrate support 110 continues to rise until thesubstrate 106, mask 108 and shadow frame 132 are disposed in theprocessing position opposite the diffuser 113.

Once the mask 108 is aligned properly, the electrode 119 is energized toeffectively fix the mask 108 relative to the substrate 106. Processingmaterials are then delivered from one or more gas sources 136 through anopening formed in a backing plate 138 while an electrical bias isprovided to the diffuser 113 to form a plasma in the processing space116 between the diffuser 113 and the substrate 106. Alternatively, aremote plasma source 140 may energize processing gas is then deliveredfrom one or more gas sources 136 to provide a plasma to the processingspace 116. Temperatures during processing may be about 80 degreesCelsius (° C.) to about 100° C., or greater.

Good contact between the mask 108 and the substrate 106 is desired inorder to control deposition of the encapsulating layers and/or toprevent a “shadow” effect at the edges of the slots 121. For example,the strips 120 should lie directly on the substrate 106 to containencapsulation material during deposition. The substrate support 110,using the electrode 119 as disclosed herein, facilitates contact betweenthe mask 108 and the substrate 106 to prevent insufficient contacttherebetween.

When there is insufficient contact between the mask 108 and thesubstrate 106, the encapsulation material may cover portions of thesubstrate 106 that are supposed to be shielded by the mask 108. However,the electrode 119 minimizes or eliminates any insufficient contactbetween the mask 108 and the substrate 106. The enhanced contact betweenthe mask 108 and the substrate 106 minimizes shadowing and/or the muraeffect which increases yield. The enhanced contact between the mask 108and the substrate 106 also enables narrow or zero bezel around OLEDdisplays on the substrate 106.

FIG. 2 is an exploded isometric view of interior chamber components usedin the chamber body 102 of the PECVD chamber 100 of FIG. 1. In FIG. 2,the substrate 106 is exploded away from the substrate support 110 butrests on the substrate receiving surface 112 of the substrate support110 during processing. The substrate support 110 is typically fabricatedfrom an aluminum material. The substrate receiving surface 112 includesan electrostatic chuck 200 (having the electrode 119 (shown in FIG. 1)embedded therein). The electrostatic chuck 200 includes the plurality ofgrooves 115 for backside gas application.

A recessed surface 202 is disposed below a plane of the substratereceiving surface 112 of the substrate support 110. The recessed surface202 includes a plurality of compressible couplers 125. In oneembodiment, each of the compressible couplers 125 may be compressiblebuttons 205. The substrate 106 is at least partially overlaid by themask 108 and the shadow frame 132 at least partially overlies the mask108. The shadow frame 132 is typically fabricated from an aluminummaterial. The mask 108 and the shadow frame 132 may include dimensionsof greater than about 0.5 meters (m) in length by 0.5 m in width.Openings 210 are shown in the substrate support 110 for access of theone or more motion alignment elements 122 (shown in FIG. 1) to extendtherethrough and contact and/or move the mask 108 relative to thesubstrate 106 to ensure proper alignment therebetween.

The mask frame 118 also includes a first side 215 on a lower surfacethereof and a second side 220 opposing the first side 215. The secondside 220 may comprise a plurality of depressions 225 that mate withprojections (not shown) on a lower surface of the shadow frame 132. Thedepressions 225 and projections (not shown) facilitate indexing andalignment of the shadow frame 132 with the mask 108. The first side 215is joined with the second side 220 by a first outer sidewall 230. Themask frame 118 also includes a raised region 235 projecting from a planeof the second side 220. The strip 120 is coupled to an upper surface ofthe raised region 235. The strip 120 may be a substantially planarrectangular member that is fastened to the mask frame 118. Collectively,the strips 120 form a mask sheet 240 having the slots 121 formedtherethrough. The strips 120 projects inwardly from the raised region235 in a plane that is substantially parallel with a plane of one orboth of the first side 215 and the second side 220.

FIG. 3 is a sectional side view of one embodiment of a portion of thesubstrate support 110 as disclosed herein. The mask sheet 240 is showncoupled to the mask frame 118, and is positioned over the substrate 106.The shadow frame 132 is shown in FIG. 3 over the recessed surface 202 ofthe substrate support 110, and a portion of the mask frame 118 ispositioned over an edge of the substrate 106. The positions of thesubstrate 106, the mask 108 and the shadow frame 132 depict a processingposition for an encapsulation process.

The substrate support 110 includes a body 300 made of a conductivematerial, such as titanium or aluminum. An electrostatic chuck 305 isdisposed on an upper surface of the body 300. The electrostatic chuck305 consists essentially of a dielectric layer (or layers) and theelectrode 119 embedded therein. The plurality of grooves 115 areembossed or otherwise formed in an upper surface of the dielectricmaterial of the electrostatic chuck 305.

The substrate support 110 also includes one of the compressible buttons205 positioned in a pocket 310 formed in the recessed surface 202. Thepocket 310 is sized to receive the mask frame 118 of the mask 108. Inone implementation, the compressible buttons 205 comprise a ceramicmaterial if the compressible buttons 205 are to have dielectricproperties. In other implementations, the compressible buttons 205 areceramic with a metallic coating or a conductive coating if a conductiveproperty is desired. The compressible buttons 205 are biased by a spring(not shown) to provide a gap 320 between surfaces of the recessedsurface 202 and the mask frame 118. The gap 320 prevents electricalcontinuity between the proximate surfaces of the mask frame 118 and thesubstrate support 110.

Additionally, the assembly shown in FIG. 3 includes a plurality ofcompressible rest buttons 315. One of the compressible rest buttons 315is positioned on the recessed surface 202 of the substrate support 110outward of the compressible button 205 and/or the pocket 310. Another ofthe compressible rest buttons 315 is positioned between the mask 108 andthe shadow frame 132. All of the compressible rest buttons 315 comprisea dielectric material, such as ceramic materials. Although only twocompressible rest buttons 315 are shown in the view of FIG. 3, aplurality of compressible rest buttons 315 are included.

FIGS. 4A-4C are schematic sectional views of portions of the substratesupport 110 as described herein. The substrate support 110 shown inFIGS. 4A-4C is similar to the substrate support 110 with the exceptionof different electrical circuitry for chucking the mask 108 using theelectrostatic chuck 305.

In FIG. 4A, the compressible buttons 205 are electrically conductivesuch that a negative voltage may be applied to the mask 108 while apositive voltage is applied to the electrode 119.

In FIG. 4B, the compressible buttons 205 are electrically conductivesuch that a positive voltage is applied to the electrode 119 and themask 108 is grounded.

In FIG. 4C, the compressible buttons 205 are dielectric such that themask 108 is electrically floating while a positive voltage is applied tothe electrode 119.

Testing of the substrate support 110 as shown in FIGS. 4A-4C yieldeddifferent results. While the electrode 119 is positively charged in allof FIGS. 4A-4C, the electrical potential of the mask 108 is different,which resulted in removing any mura effect. However, other criteria wereconsidered as detailed below.

When the mask 108 is negatively charged as shown in FIG. 4A, chuckingforce was strong (when plasma is present or not present) as compared toother configurations but de-chucking was slow as compared to otherconfigurations.

When the mask 108 is grounded as shown in FIG. 4B, chucking force wasmild (when plasma is present or not present) as compared to otherconfigurations but de-chucking was the fastest as compared to otherconfigurations.

When the mask 108 is electrically floating as shown in FIG. 4C, aminimal chucking force was observed when plasma is not present. In theconfiguration of FIG. 4C, a chucking force when plasma is present isstronger than was observed in the configuration of FIG. 4B. In addition,in the testing of the configuration of FIG. 4C, de-chucking was fasterthan the configuration shown in FIG. 4A but slower than theconfiguration shown in FIG. 4B.

FIG. 5A is a sectional side view of a portion of the substrate support110 according to another implementation. FIG. 5B is an enlarged view ofthe substrate support 110 of FIG. 5A.

In FIG. 5A, an edge 500 of the substrate receiving surface 112 of thesubstrate support 110 is shown. A portion of the plurality of grooves115 are shown on the substrate receiving surface 112 of the substratesupport 110. The edge 500 is similar about the entire perimeter of thesubstrate receiving surface 112 of the substrate support 110. Similarly,a grooved surface 505 formed by the plurality of grooves 115 issubstantially constant across the entire substrate receiving surface 112of the substrate support 110 inward of the edge 500.

The edge 500 includes a rounded corner 510. The rounded corner 510interfaces between the recessed surface 202 and the substrate receivingsurface 112 of the substrate support 110. A dielectric layer 515 isdisposed over an upper surface 520 of the body 300 of the substratesupport 110. The dielectric layer 515 includes the electrostatic chuck305 adjacent to the substrate receiving surface 112 of the substratesupport 110. The dielectric layer 515 also extends about and covers theedge 500. The dielectric layer 515 also covers a sidewall 525 of thesubstrate support 110. A plane of the sidewall 525 is generallyorthogonal to a plane of the upper surface 520. The dielectric layer 515generally electrically insulates the body 300 from the mask 108 (notshown in FIG. 5A.

In FIG. 5B, the electrostatic chuck 305 is shown on the upper surface520 of the body 300 of the substrate support 110. An interface layer 530is shown between the upper surface 520 of the body 300 and thedielectric layer 515 of the electrostatic chuck 305. The interface layer530 may include one or more layers of dielectric and/or thermallyinsulating films. The interface layer 530 may also be an anodized layerhaving a roughened surface(s) to promote adhesion between the uppersurface 520 of the body 300 of the substrate support 110 and thedielectric layer 515.

The dielectric layer 515 comprises a ceramic material, such as aluminumoxide (Al₂O₃). The dielectric layer 515 consists of a first portion 535Aand a second portion 535B. The first portion 535A is positioned abovethe electrode 119 and the second portion 535B is positioned below theelectrode 119. The second portion 535B is also sandwiched between theinterface layer 530 and the electrode 119.

The first portion 535A and the second portion 535B have a firstthickness 540 and a second thickness 545. The first thickness 540 issubstantially equal to the second thickness 545 in some embodiments. Thefirst thickness 540 is about 0.2 millimeters (mm) to about 0.4 mm, suchas about 0.3 mm in some embodiments. A thickness of the electrode 119 isabout 0.04 mm to about 0.06 mm, such as about 0.05 mm. A thickness 555of the interface layer 530 is about 0.08 mm to about 0.013 mm, such asabout 0.1 mm. A thickness 560 of the electrostatic chuck 305 and theinterface layer 530 may be about 0.7 mm to about 0.8 mm.

The grooved surface 505 includes the plurality of grooves 115 as well asa plurality of protrusions 575 formed adjacent to the grooves 115. Adepth 565 of an outermost groove 570 of the plurality of grooves 115 isabout 0.04 mm to about 0.06 mm. A height 580 of the protrusions 575 isabout 0.02 mm to about 0.04 mm.

FIG. 6 is a sectional view of a portion of the body 300 of the substratesupport 110 and the mask frame 118 showing one embodiment of acompressible button 205. The compressible button 205 includes a contactpin 600 that is movably disposed in a blind hole 605 formed in therecessed surface 202 of the substrate support 110. An elastic member610, such as compression spring, is at least partially disposed in theblind hole 605. The elastic member 610 is utilized to bias the contactpin 600 toward the mask frame 118. The elastic member 610 allows thecontact pin 600 to move toward a bottom surface 615 of the blind hole605 based on movement of the mask frame 118 while maintaining contactwith the mask frame 118. The materials of the contact pin 600 and/or theelastic member 610 may be made based on the need for electricalcontinuity or electrical insulation between the contact pin 600 and themask 108.

While the foregoing is directed to embodiments of the disclosure, otherand further embodiments of the disclosure may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A substrate support, comprising: a substrate receiving surface; arecessed portion disposed about a periphery of the substrate receivingsurface; an electrostatic chuck disposed below the substrate receivingsurface; and a plurality of compressible buttons disposed within arespective opening formed in the recessed portion that form anelectrical circuit with the electrostatic chuck.
 2. The substratesupport of claim 1, wherein the electrostatic chuck comprises adielectric layer adhered to an upper surface of a body of the substratesupport.
 3. The substrate support of claim 2, wherein the dielectriclayer comprises a ceramic material.
 4. The substrate support of claim 2,wherein an electrode is embedded in the dielectric layer.
 5. Thesubstrate support of claim 2, wherein the dielectric layer is embossedto form a groove pattern on the substrate receiving surface.
 6. Thesubstrate support of claim 1, wherein the substrate receiving surfacecomprises a groove pattern.
 7. The substrate support of claim 1, whereinthe substrate receiving surface comprises a dielectric layer.
 8. Thesubstrate support of claim 7, wherein the dielectric layer extendsoutside of the substrate receiving surface.
 9. The substrate support ofclaim 7, wherein a body of the substrate support includes a sidewall,and the dielectric layer covers a portion of the sidewall.
 10. Asubstrate support, comprising: a body comprising a conductive material;a substrate receiving surface comprising a dielectric layer adhered tothe body, wherein the body includes a recessed portion disposed about aperiphery of the substrate receiving surface; an electrostatic chuckdisposed below the substrate receiving surface; and a plurality ofcompressible buttons disposed within a respective opening formed in therecessed portion that form an electrical circuit with the electrostaticchuck.
 11. The substrate support of claim 10, wherein each of thecompressible buttons are made of an electrically conductive material.12. The substrate support of claim 10, wherein each of the compressiblebuttons are made of dielectric material.
 13. The substrate support ofclaim 10, wherein the dielectric layer comprises a ceramic material. 14.The substrate support of claim 13, wherein an electrode is embedded inthe dielectric layer.
 15. The substrate support of claim 13, wherein thedielectric layer is embossed to form a groove pattern on the substratereceiving surface.
 16. A substrate support, comprising: a bodycomprising a conductive material; a substrate receiving surfacecomprising a dielectric layer adhered to the body, wherein the bodyincludes a recessed portion disposed about a periphery of the substratereceiving surface, and the substrate receiving surface comprises agroove pattern; an electrostatic chuck disposed in the dielectric layerbelow the substrate receiving surface; and a plurality of compressiblebuttons disposed within a respective opening formed in the recessedportion that form an electrical circuit with the electrostatic chuck.17. The substrate support of claim 16, wherein each of the compressiblebuttons are made of an electrically conductive material.
 18. Thesubstrate support of claim 16, wherein each of the compressible buttonsare made of dielectric material.
 19. The substrate support of claim 16,wherein the dielectric layer comprises a ceramic material.
 20. Thesubstrate support of claim 19, wherein an electrode is embedded in thedielectric layer.